An Orthogonal Current-Reuse Amplifier for Multi-Channel Sensing

We demonstrate a micropower low-noise CMOS amplifier array that reuses bias current to improve the fundamental noise-power tradeoffs of fully-differential amplifier designs. The presented circuit implements current-reuse by stacking the differential input pairs of four amplifiers. The output drain currents of each channel´s differential pair are used as the tail currents for the differential pairs of the succeeding channel. Orthogonal current-reuse improves the noise and power tradeoff by sharing bias devices to conserve headroom. With four channels (n = 4), there are 16 unique output currents (2ⁿ) from the stack, each of which is a linear combination of the four inputs. Amplified versions of the original input signals are reconstructed by appropriately combining the small-signal output currents in output stages that operate at much lower bias currents. With an input-referred noise of 3.7 μV_rms and a bandwidth of 19.9 kHz, a single channel achieves a noise efficiency factor (NEF) of 3.0. Amortizing the bias current across the amplifier´s four channels yields an effective NEF of 1.64. The total power consumption is 15.6 μW, or 3.9 μW per path from a 1.5 V supply. Orthogonal biasing suppresses crosstalk between the channels, providing an isolation of 40 dB under 3-σ mismatch. The implemented circuit was fabricated in a standard 130 nm CMOS process and occupies an area of 0.125 mm². The proposed technique is useful for a variety of applications ranging from low-power neural recording arrays to multiphase radio baseband amplifiers.